The present invention relates to photographic printing and more particularly to an improved pin board alignment system for use with contact printing frames.
An earlier patent, U.S. Pat. No. 4,504,142, of the present inventor, describes an improved vacuum printing frame for making contact photographic prints. According to that invention, a system of inflatable bladders, connected to the blanket of the frame, pushes the mask/film sandwich against the glass top of the frame in a sequentially controlled manner. The advantage of that invention is that the air between the mask and the film is squeezed out with complete control leaving the film and mask in fully intimate contact. Furthermore, the bladder assembly can be constructed in such a way as to start the evacuation contact at the aligment pins and progressively move the contact across the mask/film sandwich.
The present invention was conceived to improve upon the existing methods of aligning the mask and film, in contact photographic printing, when precision registration between the two is required. It is particularly useful when used in conjunction with my improved vacuum printing frame, described in the earlier patent, because of the unique way the new invention takes advantage of the bladder air squeezing. The present invention will also work with conventional types of vacuum frames, including "glass down" vacuum frames.
Contact photographic printing, especially of color separations, often requires high precision alignment between the masks and the various pieces of sensitive film to be printed. The method commonly used in achieving this alignment is to punch holes in the mask and the film and place alignment pins through these holes. Two alignment pins are required for complete restriction of the relative position between the mask and the film.
There are two pin alignment techniques commonly in use. Both require a precision two-hole punch which is used to put identically spaced holes in both the mask and the film. The older alignment system uses a pin board with fixed pins spaced at the same distance as the punches in the precision, two-hole punch device. The punched film is laid down first on the fixed pin board. The mask is laid down on top of the film. The pins from the board pass through both the film and the mask to provide a reference for alignment. Additional registration holes and pins may be utilized for better control.
Once the photographic exposure is made and the film is developed, the film and mask may again be brought into precise alignment by placing both on the reference pin board. Moreover, all pieces of film which have been punched and aligned and exposed using this pin board will automatically be mutually aligned.
This traditional pin board arrangement has two significant technical difficulties. First, film expands and contracts, under thermal and humidity change, by a different amount than the expansion and contraction of the pin board. Thus, there are practical difficulties in maintaining mask/film alignment under conditions which are found in the normal working environment. Second, because the tops of the alignment pins project substantially above the top surface of the mask sheet, this type of pin board is not suitable for use with vacuum contact frames. The reason for this is that for contact printing the mask and photofilm must be pressed in firm contact with the lower surface of the glass top plate of the vacuum frame. The projecting pin ends of the traditional pin board prevent this intimate contact.
For these reasons, a second kind of pin alignment system, which uses floating pins, has been developed and is in common use with vacuum contact frames. In this system, the pin board acts only as a flat support for the mask/film sandwich. In place of fixed pins, holes are drilled through the board. The holes are somewhat larger in diameter than the alignment pins so that these pins float freely in the holes. The pins are inserted from the backside and are usually taped in place.
The pin used with this system is the standard, free pin which consists of a flat ended metal cylinder welded to a thin flat plate, which caps one end of the cylinder. The flat cap plate is about one inch square. In cross section the combination resembles an inverted "T". The flat plate permits the pin to be taped down onto any convenient flat surface.
In use, the film and mask sheets are punched with the same precision punch, which puts at least two alignment holes in each of the sheets. With pins pressed through the adjacent holes in the two sheets, each sheet of the mask/film sandwich acts as an alignment reference for the other, thereby obviating the need for the pin board to act as a position reference. Thus, the pins may float in the holes in the pin board. The advantage of this approach is that, since the mask and film are made of the same material, their expansion coefficients are the same and relative alignment is maintained over a wide range of environmental conditions.
Suppose an assembly of mask/film sandwich and floating pin board is placed in a vacuum frame. As the air is evacuated from the chamber of the vacuum frame, the board assembly is pressed firmly up against the top glass of the frame. The pins, which project up through the mask/film sandwich, press against the lower surface of the glass and are pushed down in retraction so that the flat top surfaces of the pins become flush with the top surface or the mask sheet. The mask/film sandwich is therefore able to press firmly into contact with the bottom of the vacuum frame's glass top plate. As a consequence of the pin retraction, the small metal plate joined to the bottom of the pin cylinder is pushed down into the flexible vacuum blanket which forms the bottom wall of the vacuum chamber of the vacuum frame.
The disadvantage of this commonly used type of floating pin board is that there is a substantial clumsiness which occurs when the board is used in a vacuum frame. This clumsiness occurs because the alignment pins are adhesively taped to the backside of the pin board. Each time the board assembly is used in the vacuum frame the adhesive holding tape is dislodged and the alignment pins are knocked loose. Thus, the pins must be retaped in place with each setup. This, it turns out, consumes substantial time. For this reason innovators in the graphic arts industry have been designing alternative types of pin boards.
The problem of achieving the necessary alignment stability, together with convenience of use, is not an easy one to solve. The best solution to reach the market place prior to the present invention is the Stoesser pin board. This device uses precision manufactured, telescoping pins, which consist of concentric cylinders that are spring loaded together. These "retracting" pins are permanently fastened into close tolerance wells which are precisely drilled into a thick pin board. This type of pin board is therefore similar to the first type of fixed pin board, but, because the pins can be retracted, the Stoesser board can be used with vacuum frames.
The Stoesser board has two substantial disadvantages. The first is that, because of the precision required during manufacturing, the board is expensive. The second is that the board is thick and therefore very rigid. It does not, therefore, take full advantage of the inflating bladder action of the quick pull up vacuum frame described by U.S. Pat. No. 4,504,142.
What is needed is a pin board which has some flexing capability so that as the bladders in the quick pull frame are inflated, the board conformally flexes to progressively squeeze out the air from the mask/film sandwich. The present invention solves these problems, as well as several others.
The basic solution for pin alignment results from adoption or the relative alignment, free floating pin system described above as the second basic type of pin board arrangement. One important, and non obvious, novelty introduced by this invention is the notion that the adhesive tape, which is normally used to hold the free pins to the back of the pin board, may be replaced by an elastic sheet material. The elastic sheet is bonded to the back side of a free floating type of pin board. However, the region or the elastic sheet immediately surrounding each of the pin holes is left free of bonding material. These "free regions" form pockets which contain the alignment pins. The pins are, as a result, free to "float" back and forth in the loose tolerance holes or apertures so that the mask and film can act as mutual alignment masters for each other.
The primary function of the elastic backing sheet is to act as a spring return device. A pin is permitted, by the elastic backing sheet, to yield to pressure on its top. The pin is forced downward and presses against and stretches the elastic sheet. When the pressure is released, the stretched elastic sheet provides a restoring force which causes the pin to move back into its original position. The result is that the floating pin acts as if it were spring loaded.
The elastic backing sheet yields to downward pressure on the pins in such a way that when the assembly is pressed against the glass top plate of the vacuum frame the pins are pressed down flush with the top surface of the mask sheet. In normal operation, the displacement of the pins in a downward direction is absorbed by the flexible nature of the vacuum blanket which forms the bottom wall of the vacuum chamber. In alternative embodiments, a thick, resilient backing elastic sheet could be used to absorb the downward motion of the pins entirely within its volume.
To help retain the pins in a vertical orientation, parallel to the axis of the aperture in the pin board, a metal plate that is larger than the pin cap plate is secured to the elastic sheet to underlie each pin. This backing plate enables proper orientation of the pin and enhances lateral movement of the pin with little or no frictional interference with the elastic sheet.
The top surface of the pin board is pebbled with a fine grain texture so that it may simultaneously provide a precision surface upon which the mask/film sandwich may rest and also provide channels for air to be evacuated from the space between the film and the pin board.
The preferred embodiment of the invention includes an auxiliary board which normally stays connected to the vacuum frame. This auxiliary board has a hole drilled through it which is connected to the vacuum outlet of the vacuum frame. Thus, all the air in the vacuum frame is compelled to pass through this hole in the vacuum port board when the frame is evacuated.
The vacuum port board normally is placed immediately along side the pin board next to the alignment pins. When the air is evacuated from the frame, both the vacuum port board and the pin board are pressed together against the top glass cover. The vacuum port board also has a pebble textured surface to help air to escape from the region between the top surface of the port and pin boards and the glass cover plate.
In addition, in the preferred embodiment, grooves are routed in the top surfaces of the vacuum port board and the pin board so as to provide channels for air passage. The grooves in the two boards are mated at the junction between the two boards in such a way that there is a clear air passage from the location of the pins in the pin board to the vacuum port in the port board.
The final element of the preferred embodiment of the improved pin board system is a sheet of gasket material. This sheet is substantially larger in size than the combination of the pin board and vacuum port board when the two boards are lying side by side. The excess material of the gasket sheet presses up against the glass cover plate around the edge of the two boards and provides a vacuum seal when air is evacuated through the vacuum port of the port board.
In an alternative embodiment, the gasket sheet, with pin board, assembly is used without requiring a vacuum port board. The gasket sheet has a vacuum communication hole. When the pin board is placed on the gasket sheet in such a way as to partially cover the vacuum communication hole in the gasket sheet, the hole cannot be sealed off around the edge of the pin board and there will be airflow from the top of the pin board down the edge of the pin board and into the vacuum communication hole in the gasket sheet. Thus, the gasket sheet is used with a removable pin board which does not have a vacuum port hole and which does not require a separate vacuum port board.
In other alternative embodiments of the invention the pin board may be used by itself in the vacuum frame, the pin board and vacuum port boards may be permanently joined to make a single board, or, either of these three board types may be used without a vacuum gasket sheet.